This invention relates to apparatus for abrasive blasting of the bottom of a hull of a ship in drydock. It also relates to a method and apparatus for testing the blasted surfaces for soluble chemicals.
Ship's hulls require periodic cleaning due to corrosion, paint degradation, and incrustation with marine growth such as barnacles. Periodically, a ship must be drydocked, and its hull must be blasted and painted clean. This operation is very labor intensive and it is environmentally unsatisfactory.
There are two main types of drydocks. A graving dock is a sunken pit with doors. The ship is floated into the graving dock, the doors are closed, the ship is set on blocks, and the water is pumped out from the graving dock. The other type of drydock is a floating drydock which is initially sunken. The ship is positioned over the sunken drydock, and ballast water is pumped from the drydock to raise it until it supports the ship. In either case, the ship is eventually supported on a series of blocks which can range in height from 4.5 feet to 8 feet or more.
Normally the blasting of a ship bottom is performed by men who work under the ship with blast hoses. One man using conventional equipment will, in one hour, blast around 100 to 120 square feet and he will deposit close to one-half or three-quarters ton of abrasive on the deck of the drydock. Thus, there is a large quantity of abrasive material which must subsequently be removed from the drydock floor and taken to a disposal site. This is a slow and labor intensive process, and it creates a high potential for environmental pollution.
A number of problems are associated with the blasting procedure. Space is limited between the bottom of the ship and the floor of the drydock. The blocks are an impediment to personnel and equipment beneath the ship. Vast amounts of potentially harmful dust are generated, so other workers are excluded from the vicinity of the blasting procedure.
Prior to this invention, devices for blasting vertically downwardly or horizontally have been attached to cranes or tracks. These arrangements have had some success, but they have had significant limitations. One problem has been the difficulty of maintaining a constant supply of recirculated shot for blasting. When a blaster is moved, large amounts of dust are released, particularly when the blaster has been jolted by an obstacle so that sealing engagement with the ship has been lost. Large clusters and debris are released into the air.
There have also been efforts to mechanize the abrasive blasting of ship bottoms, but none have met with widespread commercial success. A unit designed by Wheelabrator Corporation around 1972 utilized a vehicle which traveled on a drydock floor, under the hull of a ship. Abrasive particles were directed upwardly by a blast wheel and recaptured in the unit, but the unit did not have the capability of following any non-horizontal sections, it did not utilize a diverter which was repositioned in response to tilting of the blast head, and, to the knowledge of the present inventors, it did not have other significant features which are the subject of the invention claimed herein.
A submarine hull blaster was developed in the early 1980's jointly by Wheelabrator Corporation and Barnes & Reinike. This system used a Wheelabrator blast head attached to a 50 foot articulating boom. The unit was designed to blast 180.degree. of a 40 foot diameter circle. The unit was extremely large, heavy, and expensive, and it was incapable of blasting across the entire width of a ship's bottom.
Another system was developed by Hockett Systems SANDROID. It utilized complex and expensive robotic controls in order to manipulate a blast head mounted on a large industrial manlift. The vehicle was incapable of passing under a ship on five foot blocks in a drydock. The orientation of the blast head was achieved by sensors, computers, and servomotors, in contrast to the free pivot action which is used in the preferred embodiment of the present invention.
U.S. Pat. No. 4,092,942 discloses a machine particularly designed for blasting ship bottoms, but it does not utilize the concepts of the present invention. A hull-contacting "box", rather than the entire blast head was tiltable during the blasting procedure, there was no repositionable diverter in the recirculation path for abrasive, and other features of the present invention were lacking.
The apparatus of the invention recycles the abrasive material. There have been concerns that, when using recycled abrasives, some salt from the hull may be recycled onto the clean blasted steel surface where it may cause blistering or premature failure of the subsequent coating system. A major technical problem in this area has been that there has been no method of making real time measurements of the residual chloride concentration left on the blasted surface. One component of the present invention is a novel method and apparatus for making such measurements.
It is believed that the present invention satisfies a significant need in the art. The apparatus is compact and effective for performing its intended purpose. The operation of the abrasive recycling system is optimized due to an automatically repositioned diverter, and the blast head is properly positioned against the hull at a suitable orientation for effective blasting without the necessity of complex positioning devices and mechanisms. The device is able to blast extremely close to the hull-supporting blocks, the blast head motor is effectively protected by utilizing an overhung load adapter on the blast wheel shaft, the safety of the apparatus is enhanced by utilizing failsafe double acting hydraulic cylinders, and chloride measurement is effectively carried out simultaneously with the blasting operation.